An example of such detecting apparatus is disclosed in Japanese Laid-Open Patent Application No. 15,605/89 wherein a television camera onboard a vehicle takes a picture of a scene, including a road surface, located ahead of the vehicle, and image data from the camera is displayed on a picture screen defined in x-y orthogonal coordinate system. A derivative of the image data in a direction which forms an angle of 45.degree. with respect to the x-axis is calculated, and locations defined in terms of x- and y- orthogonal coordinates where the derivative exceeds a given value are selected as feature points. A succession of feature points are converted into a thin line, and a thin line or lines having a length which exceeds a given value are picked out. A thin line which satisfies a certain condition determined on the basis of features which are exhibited by a white line on the screen when it is photographed by a television camera onboard the vehicle is recognized as a white line on the road surface. In this manner, a pair of rectilinear lines which represents the white lines defining the left and the right end of a lane on which an own vehicle is running are obtained. A point of intersection between the pair of rectilinear lines is determined, and the presence or absence of an object in a triangular region defined by the pair of rectilinear lines extending between the point of intersection and the own vehicle is detected. When an object is detected, a distance from the own vehicle to such object or vehicle-to-vehicle distance if the object is a vehicle running ahead is calculated.
When the lane on which the own vehicle is running extends straightforward, any vehicle running ahead of the own vehicle on the lane exists in the triangular region. If a roof of such vehicle is displaced out of sight, the base of the vehicle lies within the region. It is to be noted that the pair of rectilinear lines which approximate the white lines on the left and the right end are determined on the basis of an image of these white lines at a location close to the camera. Accordingly, when the lane on which the own vehicle is running is curved near the own vehicle or at a point located forward of the own vehicle, the point of intersection between the pair of rectilinear lines, which can be viewed as a point of infinity for a straight lane, will be displaced to the left of a far point on this lane if the lane is curved to the right, or displaced to the right of the far point if the lane is curved to the left. Accordingly, where the road is curved, the probability that the detection of an object located ahead such as a vehicle running ahead or an obstacle located ahead, in particular, an object located far ahead fails will be high when relying upon only the search for any object in the triangular region.
Japanese Laid-Open Patent Application No. 276,214/89 proposes an automatic vehicle driving scheme in which a picture of an index along the roadside is taken by a camera onboard a vehicle to produce vehicle position information, which is compared against destination information to determine an optimum route, thus controlling a vehicle speed and a wheel steering in accordance with the route thus determined. It also proposes a two dimensional attitude changing mechanism for the camera for changing the direction, in which the camera takes a picture of the index along the roadside, in both horizontal and vertical directions. However, it will be noted that this proposal premises that the road actually exists in accordance with the optimum route determined and that there is no vehicle which is running ahead of the own vehicle on that road. In actuality, a recognition of the fact that such a road exists in fact and the reliable detection of the presence or absence of any vehicle running ahead as well as vehicle-to-vehicle distance therebetween are required.
Japanese Laid-Open Patent Application No. 124,345/94 proposes a tracking system which allows a camera to track a vehicle running ahead, by controlling the focal length and zooming so that an image of the vehicle running ahead occupies a substantially constant size on the screen, and by controlling the attitude of the camera in other horizontal and vertical directions such that the image of the vehicle running ahead is located substantially at the center of the screen. While such tracking control must premise a detection and/or identification of an initial vehicle which is running ahead, and also a search for a next succeeding vehicle when the initial vehicle running ahead which has been being tracked takes a different course and disappears from the sight forward of the own vehicle, but there is no corresponding teaching. In addition, there is no teaching about the method to deal with an erroneous tracking which may result when the vehicle being tracked takes a different course and deviates from the direction in which the own vehicle is traveling.
A pending U.S. patent application Ser. No. 08/183,369, filed by Jun SATO et al, entitled "METHOD OF DETECTING LINES APPROXIMATING AN IMAGE" corresponding to Japanese Laid-Open Patent Application No. 213,660/94 discloses an apparatus for detecting a vehicle located ahead of an own vehicle in which a camera onboard a vehicle derives an image of a road surface ahead of an own vehicle and displayed on a screen which is defined in x-y orthogonal coordinate system. A rectilinear line appearing on the screen is detected by Hough transformation, and on the basis of the line detected, a distribution on the screen of the lane on which an own vehicle is running is calculated. Subsequently, any vehicle running ahead or an obstacle located ahead on this lane is searched, and a distance to the vehicle which is found by such search is calculated.
The detection of a lane and a vehicle thereon which runs ahead of an own vehicle must cover a long distance for the search because the own vehicle is running. For example, an automatic vehicle-to-vehicle distance control may be conducted in which a vehicle running ahead is detected, a distance from the own vehicle to the detected vehicle is calculated, and a calculated distance is compared against a target distance which is established in accordance with a running speed of the own vehicle, and the own vehicle is decelerated if the detected distance is less than the target distance or the own vehicle is accelerated if the detected distance is greater than the target distance. In such instance, as a simplified example, when the own vehicle is running at a speed on the order of 40 km per hour, it may be tentatively assumed that the safety is assured if the search covers 30 m ahead. However, when the own vehicle is running at a speed on the order of 80 km, it may become necessary to detect any vehicle running ahead over a section of an increased length of 50 m or longer. In any event, because the vehicle runs at a speed over an extensive range, it is preferred that the automatic detection of any vehicle running ahead may cover a long section of 50 m or longer, assuming a high speed running.
However, it will be seen that at short distances, it is preferable to detect a vehicle with a wider angle in order to provide against lateral approach of a neighboring vehicle or to provide for a lane change, and thus it is preferred to use a camera having a wide view angle. However, this degrades the resolution for an object located at a far distance, and hence the accuracy of detection. When a narrow field of view or telescoping capability is chosen for detecting of an object located at a far distance, objects located at a near distance, in particular, a neighboring lane, will be left out, preventing a vehicle which is running on a neighboring lane from being detected. In addition, where the road is curved, a region located beyond the curvature will be removed from the screen, thus preventing a recognition of a vehicle there located.
Accordingly, while it is relatively simple to provide a tracking control of a target which is reliably identified as a vehicle running ahead on a lane on which an own vehicle is running, it is difficult to provide an extended coverage from an area located immediately in front of an own vehicle to a point far ahead which is prerequisite to such tracking control, namely, the detection of a road condition (whether it is curved or straightforward), the detection of the presence or absence of any vehicle running ahead or an obstacle located ahead, and a decision whether the detected vehicle or object is located on the lane on which the own vehicle is running. It is desirable to provide such a detection and decision technique with a high accuracy.